1. Field of the Invention
The present invention relates to an anticopying film for copying documents, consisting of a film which comprises a transparent material, has a size corresponding to that of the document to be covered or of parts thereof and, as a result of a sawtooth-like structure, has defined regions of a medium optically thinner than the film material, said regions facing the document to be covered, so that the film appears opaque in an angular range corresponding to the plan view, owing to total reflection at the saw-tooth-like interface, and appears transparent when viewed in angular ranges differing therefrom.
2. Description of the Prior Art
U.S. Pat. No. 3,887,742 describes an anticopying film of the type defined above. The film which is intended to render documents uncopiable consists of transparent plastics film which has a sawtooth-like structure. The region formed by the structure contains an optically thinner medium which is defined either by the material and document or entirely by the material. Owing to the total reflection which occurs as a result of the structure in the angular range of exposure of a copier, the film appears opaque during the copying process but transparent in an angular range differing therefrom, for example during reading.
This anticopying film has the disadvantage that the copy protection can be overcome by superposing a second anticopying film which is at least roughly coincident with the anticopying film.